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Increasing Bag Warmth

Other than the obvious, buying a warmer sleeping bag, what are some options a guy can do to use a 15-20* bag in colder temps? I can only pack in so many clothes. I have a BA Mystic SL 15. It's very possible I'll be using it come Nov in ND and it will be mighty cold. I think I read somewhere about guys using a bivy sac (??) inside of the bag to increase the warmth?

A bivy sac is used on the outside to keep you dry from rain. I don't think mine would add any warmth. Most of them don't sweat either so you would be soaked if you had it inside your bag. I bought a small fleece sleeping bag that i put inside of my mummy bag if the weather is cold. It probably only adds 5 to 10 degrees though.

I add a fleece sleeping bag to mine too. It is great in cold temps but it is heavy! I found some "backpacker" fleece bags that can be used for a bag or liner, however this one was $29. I purchased it but am not impressed with it, it seems cheap and am thinking of taking it back. It says on the label that it can give you "up to" 10* (whatever that is worth).

Now, I am thinking of going to the fabric store and buying a bulk piece of fleece. I will update when I get down there but I think that I may be able to save some weight with no zippers, cords, stitching, etc.

I have a 15 degree rated bag and bought a liner which adds up to 15 degrees of warmth. So far I haven't met an environment that is too cold for this set up, and I am cold blooded. Below is a link to the liner I have.

Thanks for the info so far guys. That Sea to Summit liner looks to be one of the more convenient options thus far, considering packability and comfort anyways. I didn't realize them bivy sacks are that heavy.

Not all bivys are heavy. I have a 9 ounce tiGoat bivy. It doesn't add warmth like the heavy USMC bivy, but it adds a few degrees of warmth, and acts as a groundcloth and spindrift protection in a floorless shelter.